1. Field of the Invention:
The present invention relates to an electrodeionization apparatus and method and, more particularly, to an electrodeionization apparatus and method incorporating specialized electroactive media to resist chemical attack, temperature degradation, and fouling, while improving electric current distribution and deionization performance.
2. Description of the Related Art:
Electrodeionization (EDI) is a process that removes ionizable species from liquids using electrically active media and an electrical potential to influence ion transport. The electrically active media may function to alternately collect and discharge ionizable species, or to facilitate the transport of ions continuously by ionic or electronic substitution mechanisms. EDI devices may comprise media of permanent or temporary charge, and may be operated batchwise, intermittently, or continuously. EDI devices may be operated to cause electrochemical reactions specifically designed to achieve or enhance performance, and may comprise electrically active membranes such as semipermeable ion exchange or bipolar membranes.
In continuous electrodeionization (CEDI), which includes processes such as continuous deionization, filled cell electrodialysis, or electrodiaresis (EDR), the ionic transport properties of the electrically active media are the primary sizing parameter. These processes are described, for example, by Kollsman in U.S. Pat. No. 2,815,320; Pearson in U.S. Pat. No. 2,794,777; Kressman in U.S. Pat. No. 2,923,674; Parsi U.S. Pat. Nos. 3,149,061 and 3,291,713; Korngold et al. in U.S. Pat. No. 3,686,089; Davis in U.S. Pat. No. 4,032,452; U.S. Pat. No. 3,869,376; O'Hare in U.S. Pat. No. 4,465,573; Kunz in U.S. Pat. Nos. 4,636,296 and 4,687,561; and Giuffrida et al. in U.S. Pat. No. 4,632,745.
A typical CEDI device comprises alternating electroactive semipermeable, anion and cation ion-exchange membranes. The spaces between the membranes are configured to create liquid flow compartments with inlets and outlets. A transverse DC electrical field is imposed by an external power source using electrodes at the bounds of the membranes and compartments. Often, electrolyte compartments are provided so that reaction products from the electrodes can be separated from the other flow compartments. Upon imposition of the electric field, ions in the liquid are attracted to their respective counterelectrodes. The compartments bounded by the electroactive anion membrane facing the anode and the electroactive cation membrane facing the cathode become ionically depleted, and the compartments bounded by the electroactive anion membrane facing the cathode and the electroactive cation membrane facing the anode become ionically concentrated. The volume within the ion-depleting compartments, and preferentially within the ion-concentrating compartments, is also comprised of electrically active media. In continuous deionization devices, the media may comprise intimately mixed anion and cation exchange resins. The ion-exchange media enhances the transport of ions within the compartments and can also participate as a substrate for controlled electrochemical reactions. The configuration is similar in electrodiaresis devices, except that the media comprise separate, and sometimes alternating, layers of ion-exchange resin. In these devices, each layer is substantially comprised of resins of the same polarity (either anion or cation resin) and the liquid to be deionized flows sequentially through the layers.
A number of CEDI devices and processes have been successfully commercialized, for example, as disclosed by Giuffrida in U.S. Pat. No. 4,298,442; Giuffrida et al. in U.S. Pat. No. 4,632,745; Siu et al. in U.S. Pat. No. 4,747,929; Palmer in U.S. Pat. No. 4,804,451; Giuffrida et al. in U.S. Pat. No. 4,925,541; Giuffrida U.S. Pat. Nos. 4,931,160 and 4,956,071; White in U.S. Pat. No. 5,116,509; Oren et al. in U.S. Pat. No. 5,154,809; Giuffrida et al. in U.S. Pat. No. 5,211,823; and Ganzi et al. in U.S. Pat. Nos. 5,308,466 and 5,316,637. In addition, there have been a wide range of devices described in the literature, including Walters et al., "Concentration of Radioactive Aqueous Wastes," Ind. Eng. Chem., Vol. 47, 1, pp. 61-67 (1955); Sammon et al., "An Experimental Study of Electrodeionization and Its Application to the Treatment of Radioactive Wastes," AERE-R3137, Chemistry Division, U.K. AEA Research Group, Atomic Energy Research Establishment, Harwell (June 1960); Glueckauf, "Electrodeionization Through a Packed Bed," British Chemical Engineering, pp. 646-651 (Dec., 1959); Matejka, "Continuous Production of High Purity Water by Electrodeionization," J. Appl. Chem. Biotechnol., Vol. 21, pp. 117-120 (April, 1971); Shaposhnik et al., "Demineralization of Water by Electrodialysis with Ion-Exchanger Packing Between the Membranes," Zhurnal Prikladnoi Khimii, Vol. 46, 12, pp. 2659-2663 (December, 1973); Komgold, "Electrodialysis Processes Using Ion Exchange Resins Between Membranes," Desalination, Vol. 16, No. 2, pp. 225-233 (1975); and Kedem et al., "Reduction of Polarization by Ion-Conduction Spacers," Desalination, Vol. 27, pp. 143-156 (1978).
There remains a need for devices and processes with improved reliability and the ability to operate under more rigorous conditions with reduced power consumption and reduced membrane area. Often, the limiting factor in the applicability of CEDI is the ability of the electroactive media within the device to withstand the temperature, chemical, and fouling conditions of the liquid to be processed. One difficulty in specifying such electroactive media results from the need in most applications to incorporate both anion and cation media within the compartments and the membranes. Many times, conditions are relatively benign for media of a given fixed charge, but are limited by the oppositely charged media. For example, the presence of iron in the liquid to be processed may result in fouling of cation exchange resin and membrane, but would not affect the performance of anion exchange resin or membrane. Conversely, the presence of high temperature, chlorine or intermediate molecular weight weak organic compounds may result in degradation, oxidation, or fouling of anion exchange resin and membrane, but would not affect the performance of cation exchange resin or membrane.
In other cases, performance of CEDI is limited by difficulty in providing an effective cleaning regimen when a device is fouled or scaled with precipitate. Strategies for cleaning include methods for introducing cleaning chemicals into the device is disclosed, for example, by Giuffrida et al. in U.S. Pat. No. 4,753,681. This strategy is often limited by the chemical or temperature resistance of one of the electroactive components of the device. In other instances, there is no cleaning chemical that is effective in completely removing a foulant from the active media (a root cause is the common practice of incorporating media with a permanent ionic charge). Independently, or in combination with chemical cleaning, other defouling and descaling strategies include the practice of reversing polarity of the external power source as described, for example, by Giuffrida et al. in U.S. Pat. No. 4,956,071 and Katz et al. in U.S. Pat. No. 5,026,465. Although sometimes effective, this practice is also limited by the incorporation of low temperature resistance, chemical resistant media, or media of permanent fixed charge.
Performance of CEDI is may further be limited by difficulty in obtaining the desired electrical current distribution with the device. Electroactive media of permanent charge may change their electrical resistance properties in undesired ways depending on their ionic form. For example, in the ion substitution of sodium with hydrogen ion in EDR, most cation exchange resin will preferentially transport hydrogen ion over the desired transport of sodium ion. This results in electrical inefficiencies and, under certain circumstances, may cause pH shifts that are detrimental to valuable products within the liquid. In another example, a given electroactive media may be desirable for transport properties, such as the Type II anion membrane and resins for continuous deionization and EDR, but may have the undesirable properties of catalyzing the ionization reaction of water to hydrogen and hydroxide ions.
Furthermore, the presence of gases, poor flow distribution, low temperature and/or low conductance liquids within the electrolyte compartments may be detrimental to electrical current distribution, thereby reducing the efficiency of deionization. However, as a result of the oxidizing or reducing conditions common within the electrolyte compartments, standard electroactive media such as ion exchange resins or activated carbons (e.g. carbons prepared by pyrolyzing coal into small imperfect granulars having an interfacial area on the order of 10 m.sup.2 /g and contain up to 20% ash impurities) cannot be incorporated within the electrolyte compartments because of their limited chemical resistance.
In electrochemical ion exchange (EIX) and capacitive deionization (CapDI), both the transport and the capacity of the electroactive media are important sizing parameters. In EIX, the electrode reactions produce ions that are used for ionic substitution reactions within the electroactive media Typical EIX devices, as described, for example, by Hobro et al. in "Recycling of Chromium from Metal Finishing Waste Waters Using Electrochemical Ion Exchange," Electrochemical Society, Symposium on Water Purification, PV94-19, pp. 173-183 (1994), may or may not comprise ion exchange membranes. Similar to the CEDI processes, the performance of the EIX devices are often limited by use of permanently charged media, media of limited temperature and/or chemical resistance, and/or media with undesirable ion transport properties. In CapDI, high surface area, usually carbon type electrodes are used to adsorb ions as polarity is imposed, and then desorb ions as the electric field is removed or reversed. As with other processes, typical CapDI devices may also be limited by the use of electrodes of low chemical resistance, or limitations of the media to act equally well or in the desired manner when in use as both a cation and anion adsorber or as both an ion adsorber and desorber.
A need therefore remains for an improved electrodeionization apparatus which provides electroactive media that is resistant to chemical attack, temperature degradation, and fouling. In addition, a need remains for an electrodeionization apparatus having improved electric current distribution and deionization performance. The electroactive media should have the ability to change its charge and transport properties when in the presence of permanently charged media, varied chemical or electrochemically induced environments, and/or the presence, absence, or reversal of an externally imposed electrical field. Moreover, there is a need for an electrodeionization apparatus which promotes or limits electrochemical water splitting depending on its chemical environment. Lastly, there is a need to provide improved methods of operation of an electrodeionization apparatus.